Ground water represents greater than 95% of the available fresh water in the United States including the Great Lakes (Faust and Aly, 1987). Contamination of ground water resources with persistent contaminants such as toxic metals, MTBE, and chlorinated solvents is a growing problem. Ground water use by public utilities tripled between 1950 and 1975 and from 3.5 bgd to 11 bgd and use continues to grow rapidly in recent years (Faust and Aly, 1987). Detection of organic contaminants such as chlorinated solvents in municipal supply wells is becoming more and more common. xe2x80x9cHalogenated volatile organic compounds (VOCs), including chlorinated aliphatic hydrocarbons (CAHs) are the most frequently occurring type of contaminant in soil and ground water at Superfund and other hazardous waste sites in the United States. The U.S. Environmental Protection Agency (EPA) estimates that cleanup of these sites will cost more that $45 billion (1996 dollars) over the next several decades (EPA, 1997). Innovative technologies, including in situ bioremediation, are being developed and implemented in an effort to reduce the cost and time required to clean up those sites. In situ bioremediation is increasingly being selected to remediate hazardous waste sites because, when compared to above-ground technologies, it is usually less expensive, does not require waste extraction or excavation, and is more publicly acceptable as it relies on natural processes to treat contaminants.xe2x80x9d (EPA 542-R-00-008, July 2000).
Natural attenuation of chlorinated solvents by reductive dechlorination often occurs at sites where an electron donor (food source or substrate for microbes) is present along with the chlorinated solvent contamination. As dissolved oxygen and other electron acceptors become depleted some microbes are capable of using the chlorinated solvents as electron acceptors. For selected compounds such as chlorinated ethylenes sequential dechlorination to a harmless byproduct ethylene can be achieved under favorable environmental conditions (EPA/600/R-98/128 September 1998).
In recent years efforts have been made to produce this anaerobic treatment effect by injection of electron donor into the subsurface. An overview of these technologies can be reviewed in the EPA document Engineered Approaches to In Situ Bioremediation of Chlorinated Solvents: Fundamentals and Field Applications (EPA 542-R-00-008 July 2000). Other inorganic and organic compounds can be degraded or immobilized under anaerobic conditions including selected toxic metals, nitrate, and MTBE. For sites that do not have sufficient amounts of natural electron donors to drive anaerobic natural attenuation, injection of microbial substrates has proven to be a cost-effective treatment or plume migration control measure. The microbial substrates can be injected into the contaminant source area where residual contamination is adsorbed onto soils or injected in a line across a ground water contaminant plume to form a permeable reactive wall to prevent further contaminant migration.
A wide variety of sugars, alcohols, organic acids, and even molecular hydrogen have been used successfully as electron donors to enhance anaerobic biotransformation processes. Most of these compounds are rapidly consumed after injection and must be replaced by either continuous low concentration delivery systems or with frequent batch additions of additive solution. Contaminant source areas can not be effectively removed or even precisely located for many ground water contaminant plumes. The presence of residual chlorinated solvents adsorbed onto soils or present as dense nonaqueous phase product (DNAPL) serves as an example of persistent ground water plume source areas that can last for many decades. These persistent contaminant source areas continue to contaminate ground water for many years such that continuous operation of recirculation systems or frequent substrate injections can be very costly over the life of a project. Long-term injection of substrates into wells or infiltration galleries often leads to severe bacterial fouling problems adding to project operation and maintenance costs.
Recent interest has developed in the use of materials that slowly biodegrade or slowly release organic matter into ground water over time. A commercial product marketed under the brand name of HRC (Hydrogen Release Compound) has been used at many sites to slowly release lactic acid. As described in U.S. Pat. No. 6,420,594 such compounds comprise esters of polylactic acid and polyols. The compound is relatively expensive (approximately $6.00/pound) compared to common low molecular weight substrates such as sugars (approximately $0.30/pound), but is designed to provide a steady supply of lactic acid to ground water for up to a year. Injection of HRC into soil or ground water often results in a limited treatment radius due to the small volume and high viscosity of the injected material. Although the longevity of the product is greatly improved over simple sugars or other low molecular weight substrates reinjection of the product on an annual basis makes this treatment cost prohibitive at many sites.
A variety of sparingly soluble materials also have been explored for their potential to produce a long-term release of electron donors including canola oil, soybean oil, and oleate. A variety of vegetable oils have been demonstrated to be effective electron donors to stimulate anaerobic biodegradation. Although edible oils such as soybean oil have a much lower viscosity than a semisolid product like HRC, distribution in saturated soils is difficult. Soybean oil has a viscosity approximately 80 times higher than water, which results in multiphase fluid flow and potential oil blockage of soil porosity.
The problem of oil viscosity can be greatly reduced by injecting the edible oil as an oil/water emulsion. If the oil droplets are made small enough a dilute but efficacious emulsion has a fluid viscosity that is essentially the same as water. A comparison of the effectiveness of pure soybean oil injection with emulsified oil injection was completed at five Air Force sites, with data clearly indicating that the performance of the emulsified oil is superior.
Injection of pure oil or large droplets of emulsified oil blocks soil pores producing treatment zones that are ineffective because they prevent free flow of ground water through the oil treated area. Injection of pure soybean oil into porous soil media has been shown to reduce water permeability by up to 100%. When oil droplets were reduced to a 6.1-micron median droplet size the soil permeability was still reduced by up to 70% in a quartz sand soil. By reducing the median droplet size of an oil/water emulsion to 2.1 microns the loss of soil permeability from pore clogging was reduced to 25% such that 75% of the original permeability remained after oil/water injection.
Published experimental data suggests that oil/water emulsions in porous media are best described by a filtration model rather than an oil droplet retardation model. Soil column studies confirmed that oil droplets that are trapped within a soil matrix are not easily released, even when the columns are flushed with several pore volumes of clean water. Oil droplets can be retained in porous soil media by two mechanisms straining and interception. In the straining process droplets that are larger than a pore throat physically lodge in the pore throat. In the process of interception droplets contact soil particles and attach to the soil matrix because of electrical charge, Van der Waals forces or other physical mechanisms.
Both mechanisms can act to block a soil matrix pore throat even when oil droplets are smaller than the soil pore throats. As an example a 2.5 micron pore throat may be blocked by 1.0 micron oil droplets if both sides of the pore throat soil particles first attract oil particles by interception, followed by a third particle intercepted by straining in the now reduced pore throat size. As the smaller diameter pore throats become blocked flow is diverted preferentially to larger pore throats within the soil matrix.
It has been stated in the literature that to prevent a large reduction of soil permeability an oil emulsion must have a median oil droplet size to median soil pore throat size ratio of 0.3 or smaller. Unless the emulsified oil injection is conducted on coarse soils it is critical that the median oil droplet size be under 1 micron or substantial loss of soil permeability will result rendering the treatment ineffective. This is especially critical for permeable reactive wall installations where a loss in soil permeability will result in ground water flowing around the reactive wall untreated.
A small droplet size is also beneficial in preventing gravity separation of oil and water from an oil/water emulsion. As predicted by Stokes law an emulsion with smaller droplets will separate more slowly than an emulsion with larger oil droplets proportional to the square of the ratio of the droplet size. An emulsion with oil droplets of one micron will take approximately 100 times longer to separate by gravity than an emulsion containing droplets 10 microns in diameter.
There are several potential methods for preparing an oil/water emulsion for remediation. The simplest method is to use a large amount of surfactant and stir the solution with a low-shear mixer. Emulsions produced in this manner require a large amount of surfactant which greatly increases the risk of mobilizing contaminant free product and produce coarse emulsions with many oil droplets of 20 microns or greater produced.
Emulsions with smaller droplet sizes can often be prepared by adding more physical energy to the emulsion through the use of a high shear mixer. The use of very-high shear mixing equipment such as a dairy homogenizer for field emulsification is difficult because of high equipment capital costs, large size and weight of the equipment, and excessive electrical power requirements.
The use of a portable slotted head high-shear, inline mixer has been suggested for field applications. Although the high-shear slotted head inline mixer appears to be a substantial improvement over low-shear mixing, such field emulsification is time consuming, cumbersome, and does not produce as high a quality emulsion as off-site processing. Multiple passes through the slotted head high-shear, inline mixer produced a median droplet size of approximately 5 microns. As previously described an oil particle size of 5 microns could still cause substantial pore throat blockage and reduced permeability in fine grained soils. For this reason a 5-micron median droplet size was deemed unacceptable for remediation purposes.
It is an object of this invention to provide a method for the long term anaerobic remediation of contaminated soil and ground water using an aqueous emulsion of a sparingly soluble, digestible fat or oil.
It is another object of this invention to provide an aqueous emulsion of a sparingly soluble fat or oil effective for remediation that has a minimal increase in viscosity over water and causes minimal pore blockage in soil.
Still another object of this invention is to provide an aqueous emulsion of a sparingly soluble fat or oil of superior stability and droplet size for long term remediation of contaminated soil and ground water.
Yet another object of this invention is to provide a method for the remediation of contaminated soil that employs more than one anaerobic electron donor and nutrient.
Still another object is to produce remediated soil and ground water in which contaminant levels have been reduced below recognized safety levels.